This invention relates to a method of fabricating an optical fiber base material by a modified CVD method (MCVD method, modified chemical vapor deposition method).
When an optical fiber base material is fabricated by an MCVD method to provide a graded-index (GI) type quartz optical fiber, the method has the steps of concentrically accumulating synthetic glass on the inner peripheral surface of a quartz tube to form predetermined glass layers, then collapsing the quartz tube to strongly heat and shrink the tube and then filling the tube to form solid state as a rod-shaped base material.
The glass layers accumulated in the quartz tube are made of germanium-doped quartz for a core. When collapsing the quartz tube after accumulating the glass in the above method, Ge content in the inner periphery of the glass layer is evaporated by the strong heating at the collapsing time, and becomes silica-riched, thereby causing the refractive index of the optical fiber base material which is referred to as "center-dip" to become abnormal, i.e., causing the refractive index of the material from being deteriorated in the central portion. Thus, the refractive index of the resulting optical fiber abnormally decreases.
The center-dipped optical fiber thus produced has, as pointed out, a decrease in the baseband characteristic.
In order to eliminate such problems, a glass material and a doped material are supplied in gaseous state into the quartz tube and the tube is strongly heated to sufficiently reduce the diameter in the remaining space in the tube and the tube is then collapsed as disclosed in Japanese Patent Laid-open No. 130131/1983.
According to this conventional method, the dopage of Ge is affected by the temperature and the composition of the core glass. Thus, it is necessary in this method to control the supply of the raw material. In this case, it is not easy to correct the dopage of Ge by applying thermal load to shrink the glass. Therefore, according to this method, it is difficult to provide the base material having small center-dip and a predetermined refractive index distribution.